1. Field of the Invention
The present invention relates to cardiopulmonary resuscitation and, in particular, to a system for generating cyclic fluctuation in intrathoracic pressure for use in cardiopulmonary resuscitation and non-invasive circulatory assistance.
2. Description of The Related Art
Cardiac arrest is generally due to ventricular fibrillation, which causes the heart to stop pumping blood. The treatment of ventricular fibrillation is defibrillation. If, however, more then a few minutes have lapsed since the onset of ventricular fibrillation, the heart will be sufficiently deprived of oxygen and nutrients such that defibrillation will generally be unsuccessful. Thus, it is necessary to restore flow of oxygenated blood to the heart muscle by cardiopulmonary resuscitation in order for defibrillation to be successful.
It is known that fluctuations in intrathoratric pressure can produce blood flow during cardiopulmonary resuscitation. Thus, efforts have been made to increase intrathoratric pressure to levels above those obtained conventionally in order to produce increased blood flow. For example, high-pressure ventilation has been used with simultaneous mechanical sternal compression or circumferential thoracic compression with an inflatable vest, to increase the levels of intrathoratric pressure generated. However, such techniques have required endotrachacheal intubation and this invasive technique and the simultaneous high-pressure ventilation have presented high risks to the patient and are cumbersome and time consuming and thus have limited usefulness.
In accordance with a technique of circulatory support intrathoracic pressure changes phase locked to the cardiac cycle have been used to assist the failing but still beating heart. These intrathoracic pressure changes have been generated either by lung pressurization simultaneous with chest compression or by lung pressurization with the chest bound to prevent thoracic expansion. In all cases, then, pressurization of the lungs was required to produce adequate changes in intrathoracic pressure.
Furthermore, we previously developed a system that could generate large changes in intrathoracic pressure without simultaneous ventilation. That system used a thoracic vest that was rapidly inflated and deflated. However, the vest had to be applied to the patient so that it was extremely tight about the chest and had to be positioned very accurately in order for it to function properly. Therefore, because the vest had to be attached so tightly it compromised ventilation and the tightness and positioning requirements made it very difficult for the vest to be applied correctly. If the vest was not applied correctly, higher pressures in the vest had to be used in order to obtain a given level of intrathoracic pressure. Higher vest pressures however would lead to excessive trauma to the patient thereby compromising resuscitation.
As is apparent from the foregoing it can be appreciated that the success of resuscitation is directly related to the generated intrathoracic pressure and inversely related to the amount of trauma produced.
As noted above the earlier techniques requiring endotracheal intubation were difficult to apply properly and the high-pressure ventilation of the lungs could damage them. Furthermore, our earlier system produced inconsistent levels of intrathoracic pressure during assistance of a beating heart if the heart rate was irregular. These inconsistent levels of intrathoracic pressure could result in pressures that were far too low for adequate assistance or that were high enough to cause excessive trauma.
It would therefore be desirable to provide a system which can generate high levels of intrathoracic pressure without the need for simultaneous ventilation through endotracheal intubation. Preferably, such a system would generate a maximum fluctuation in intrathoracic pressure, adequate ventilation and would be safe to the patient and easy to implement.